In order to analyze a spectrum of light, spectroscopic devices are used. The scientific, technical and industrial applications are numerous. The testing and verification of materials, the detection of chemical or biological species, etc. may be mentioned. In order to carrying out a spectroscopic analysis, there exist two main types of analysis systems. The first systems are based on chromatic dispersion of light. The light to be analyzed is decomposed by a dispersion system (dichroic plates, prisms, diffraction gratings, etc.) and sent onto a certain number of sensors, each sensor being dedicated to a given spectral band. The second systems operate by interferometry. The light to be analyzed passes through an interferometer (Fizeau, Michelson, etc.). The interferogram produced by the interferometer allows the spectral distribution of the light to be recovered by Fourier analysis.
The most common detection comprises only three detectors working within three different spectral bands but with some overlap. Information, referred to as calorimetric information, is thus recovered allowing the ‘color’ of the light to be determined. This method is universally applied to the detection and to the formation of colored images insofar as it is possible to integrate a large number of calorimetric sensors. Of course, this information, sufficient for determining the color, is not sufficiently precise for determining even approximately the spectral distribution. Thus, two very different spectra can have the same color.
Furthermore, there exist multispectral detection systems allowing, even rudimentary, spectral information to be obtained with a structure that is sufficiently simple and of limited size in order to be compatible with a matrix arrangement and parallel operation allowing imaging applications. Several approaches have been proposed. Thus, the U.S. Pat. No. 6,465,860 describes a microelectronics device composed of successive layers of semiconductor materials having different absorption spectra, each absorbing layer being separated from the following one by an insulating layer. The fabrication of this type of device is necessarily complex in the sense that each absorbing layer has to have an absorption different from that of other layers and has to be perfectly calibrated.
The patent FR 2 879 287 proposes a second approach. The device described in this patent is a spectroscopic detector comprising a single-mode waveguide one of the faces of which comprises a mirror. The light to be analyzed enters via the opposing face, and is reflected on the mirror thus creating a stationary wave inside the waveguide by the Lippmann effect. Indeed, when an incident wave is reflected on a mirror, it interferes with itself. The interferogram obtained in the structure is representative of the spectral distribution present in the incident wave. The evanescent waves created by this stationary wave are picked up by local detectors placed at the periphery of the waveguide. The analysis of the signals coming from these detectors allows the spectrum of the light to be recovered. It is clear that this principle only works well in the case of a single-mode waveguide. However, the implementation of systems for detection of evanescent waves at the periphery of a single-mode waveguide whose dimensions are less than the interfringe distance equal to λ/2n, λ being the wavelength of the incident wave and n being the optical index of the material, poses considerable technological problems when the wavelengths become optical wavelengths.